Historically modulation of engine output torque in a vehicle was solely by control of accelerator pedal position, which in turn usually determined the position of a throttle valve at the entrance to an air inlet manifold. The response of an engine to a change of throttle valve position is generally delayed by the volume of air in the inlet manifold (upstream of the inlet valve(s)), so that several combustion cycles may pass before output torque is modified in the intended direction. This delay is generally not noticed by vehicle drivers, or is accommodated by an alteration of driving style.
More recently aspects of air management, fuel management and combustion timing have been under the control of an electronic control unit (ECU) of the engine, so that many kinds of operating conditions can be accommodated, to the intent that the engine runs efficiently at all conditions of speed and load. In particular, torque change requests from other vehicle systems, such as chassis, transmission and braking systems, must be prioritised and acted upon. A more rapid means of modulating engine output torque was considered desirable, and as a result active variation of the timing of an ignition spark was proposed. Active control of ignition timing permits successive combustion events in an individual combustion chamber to be timed differently, and furthermore permits different combustion chambers of a multi-cylinder engine to be timed differently; this method of control is significantly more rapid than one based upon throttle valve movement.
By active control of ignition timing, a torque down request, for example during a speed ratio change, can be effected rapidly by retarding the timing of an ignition spark. Output torque is reduced, but as a result combustion is relatively inefficient. Typically additional waste heat is generated in the engine and in the engine exhaust system. Inefficient combustion may also lead to an increase in undesirable exhaust emissions.
In anticipation of a torque-up request, for example when the engine is idling, an excess of air/fuel mixture may be introduced into a combustion chamber and ignited with a retarded spark so as to produce the torque output required to achieve the desired idling speed. A rapid response to a torque-up request is achieved by advancing the timing of an ignition pulse between one combustion event and the next. However it will be appreciated that the technique also relies upon inefficient combustion, with the disadvantages noted above.
It is also known to vary operation of the inlet and/or exhaust valves to obtain a variation of the volume of air available for combustion, but such systems relying on movement of cam elements are generally considered to be slow, and about an order of magnitude slower than is provided by a variation in the timing of an ignition spark.
What is required is a means of continuous modulation of engine output torque which is capable of a response as fast as variation of ignition timing, but without the associated inefficiencies of combustion.